Carcinoma in situ of the testis is an abnormality of the seminiferous epithelium characterized by aneuploid cells with clear cytoplasm located along the tubular basement membrane or in the lumina of tubules. Morphological, cytogenetic, and histochemical features of this anomaly are presented. An adequate fixation of testicular tissue samples is necessary for correct diagnosis. The fact that approximately 50% of carcinoma in situ may develop into malignant germ cell tumours of the testis in 5 years and the investigations of the testicular tissue adjacent to these tumours support the malignant nature and invasive potential of testicular carcinoma in situ. The incidence of carcinoma in situ in persons at risk is reviewed.